Electrochemical gas sensors usually have a housing with at least one measuring electrode and a counterelectrode in an electrolyte and with electric connection lines from the electrodes to a measuring unit arranged, in general, outside the housing, wherein the measuring unit is usually a potentiostat with an evaluating circuit. The connection lines are contacted with the electrodes in the known manner in the form of drain wires made especially of precious metals such as platinum, because these are resistant to the common electrolytes, such as acids, alkalies and salt solutions.
However, a number of drawbacks are also associated with the use of precious metal drain wires. The catalytic activity of the precious metals is utilized in the majority of electrochemical gas sensors for detecting measured gases. These gas sensors are not selective but always detect a plurality of gases due to the high catalytic activity of their measuring electrodes. For the same reason, these sensors have high basic currents as well as signals for air humidity and temperature changes.
The precious metal measuring electrodes of these gas sensors usually have very large active surfaces, on which the reaction of the measured gas or measured gases takes place. The surface of the likewise catalytically active contacting wires, which is small compared to this, therefore makes a nearly unmeasurable contribution to the signal current.
Sensors which no longer detect the gas to be measured directly at the measuring electrode but indirectly via a chemical mediator were therefore developed (DE 19939011 C1) in the trend towards the more sensitive and selective detection of measured gases. Measuring electrodes comprising diamond-like carbon (DLC) are used here, which do not have any catalytic activity, show no cross sensitivity, have extremely low basic currents, and have no signals for a change in the air humidity and temperature. The last, catalytically active component in the area of the measuring electrodes of these gas sensors is the precious metal drain wire, which, acting practically as an electrode, now represents the essential disturbance in respect to the above-described problems.
Even though this situation can be improved by the use of graphite, carbon fibers or glass carbon for contacting the measuring electrode, it cannot be solved, because these materials also have some interfering cross sensitivities, e.g., for nitrogen oxides. Moreover, these materials, which are in contact with the electrolyte, are oxidized over time and lose contact with the measuring electrode.
However, there also are problems concerning the tightness of the connection lines through the housings of such electrochemical gas sensors because, for example, oxygen is formed at the counterelectrode and the contacting wire thereof in oxygen sensors, so that a tightness problem develops at the site at which the contact wire is led through the housing because the plastic surrounding the metal wire and thus exerting a sealing action is progressively degraded by oxidation due to the evolution of gas and the electrolyte can flow out of the housing along the gap thus formed.
Another cause of leaks is the different coefficients of expansion of precious metal wires and the plastic housing, so that capillary gaps, through which the electrolyte can flow out, are formed over time at the openings through which the wire is led out of the housing for this reason as well. To overcome these drawbacks, special liquid and labyrinth sealing systems are used, which are associated with additional working steps and material consumption and continue to cause problems in practice because of the different materials used and the temperature changes occurring.
The alternative use of a conductive composite plastic injected into the sensor housing as a substitute for the draining connection lines has also failed to prove successful because the electric conductivity of these plastics declines over time since the components responsible for the conductivity, such as graphite or carbon fibers, undergo oxidation and thus develop a high ohmic resistance.
US 2005/0186333 A1 discloses an electrochemical measuring arrangement for determining an analyte in an aqueous liquid test sample, in which the measuring electrode comprises an electrically conductive film, which is formed with carbon nanotubes.